Pulse height analyzers frequently function by counting the pulses falling within a particular energy or voltage range, called a "window." Two comparators generally set the upper and lower limits of the range, with the subsequent cicuitry providing a single pulsed output for each input pulse in the range.
One area in which such pulse height analyzers have found appreciable use involves the counting of decay products of radio isotopes. Devices performing the function generally include a transducer which produces electrical pulses having magnitudes generally related to the energies of the decay products.
The window feature of the analyzer has particular value in discriminating between the particles emanating from a particular nucleus as opposed to those from other sources. The particles produced by a selected radio isotope undergoing decay will generally have energies falling within a narrow range. An appropriate selection by the comparators of the upper and lower limits of the analyzer's window permits the counting of these particles and the rejection of others with different energies.
Pulse height analyzers have had to overcome a problem associated with the use of the two comparators to establish the desired range. The pulses provided by the transducer almost invariably display sloping beginning and ending edges as opposed to square edges. Accordingly, a pulse exceeding the levels set by both comparators, and thus falling outside the energy range, effects transitions of the two comparators at different times. Consequently, analyzers of this type require circuitry to compare events occurring at different times. The circuitry must also, of course, accomodate the desired pulses which induce a transition in one of the comparators but none in the other.
Previous analyzers have extended the responses of the comparators to an input pulse sufficiently to insure coincidence of the extended responses at some particular time. The remaining circuitry of the analyzer then compares these extended responses. It determines whether the pulse exceeded the levels established by both comparators, thus falling outside of the established range, or merely exceeded the lower level to come within the selected window.
The analyzers have extended the comparators' responses by including resistance-capacitance (R-C) components within the circuitry. The monostable multivibrator represents one method of including the desired R-C components as shown in J. Laughter's article "TTL Gates Speed Up Pulse-Height Analyses" appearing in the Aug. 14, 1972, issue of Electronics on page 111.
Although performing satisfactorily in a number of applications, the circuits employing the R-C components, however, possess a number of inherent limitations. The proper comparison of the two comparators' outputs requires fairly precise timing on the extensions produced by the components. This in turn imposes strict limitations on the actual value of the resistances and compacitances used since these determine the relevant periods of extension. Furthermore, since these components tend to change over a period of time, they must undergo checks and possible replacement to assure that the originally selected values have not undergone alteration or degeneration.
Moreover, extending the outputs of the comparators increases the time that the circuitry remains occupied with a particular pulse. During this time, it can not react separately to a subsequent pulse. Thus, extending the comparators' outputs limits the ability of the pulse-height analyzer to resolve, or separately count, closely spaced pulses.
Additionally, the circuit makes its determination at a predetermined time after the lower-level comparator has signaled the arrival of a pulse. However, large pulses with a very slow rise time, may not have yet exceeded the upper level at this preset time. As a result, the circuit will indicate a pulse within the seleced energy range although, subsequently, the pulse exceeds the upper level. Thus, the circuit shows a limitation with very slow rising pulses as with very fast pulses. Because of the above limitations, the search continues for pulse-height analyzers with less severe restrictions.